Automatic volume control circuits



Sept. 29, 1936. E. T. DTcKEY AUTOMATIC VOLUME CONTROL CIRCUITS Filed March `6, 1955 TNVENTOR EDWARD T. DTCKEY BY f ATTORNEY Patented Sept. 29, 1936 UNITED STATES AUTOMATIC VOLUME CONTROL CIRCUITS Edward T. Dickey, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application March 6, 1933, Serial No. 659,781

4` claims;

My present invention relates to a novel and improved method of obtaining automatic volume control in radio receivers without recourse to changing the characteristics of the amplifier tubes.

In general, there are two types of automatic volume control arrangements adapted for use with radio receivers. In one of these types the characteristic of one or more of the tubes of the receiver, usually the amplifier, is regulated. In the other type the transmission of signal energy through the receiver network is regulated by controlling the high 'frequency coupling between two designated points in the receiver network. As is well known to those skilled in the art, each of these types of automaticy volume control is accompanied by certain advantages and disadvantages. The form of volume control arrangement involving the changing of certain of the operating characteristics of the tubes intended primarily for amplifier purposes has a numberof disadvantages,among which are tendencies to produce cross-modulation and harmonics in the radio frequency amplifier.

Accordingly, it may be stated that it'is one f the main objects of my present invention to provide an improvement in Volume control arrangements for radio receivers, and particularly in automatic volume' control arrangements, wherein, instead of changing the characteristics of the amplifier tubes, the transfer of radio frequency energy through a coupling capacity is controlled by a space discharge device functioning in cooperation with said coupling capacity as a variable high frequency energy shuntingY impedance.

The aforementioned coupling capacity may comprise a coupling condenser between two successive radio frequency energy points of the receiver, but it may also consist of an inherent inter-electrode capacity of one of the tubes of the receiver. Therefore, itv may be stated that it is another important object of the present invention to provide an automatic volume control arrangement for a radio receiver wherein a radio frequency coupling capacity is used as the 'volume control instrumentality, the regulation of the transfer of energy through this capacity being accomplished by connecting to the capacity a variable shunting impedance, such as a space discharge device, and the conductivity of this impedance being automatically controlled by the direct current component of a rectifier plate current.

Another objectof the present invention is to (Cl. Z50-20) provide in a radio receiver equipped with a recti-v fier, such as the detector of the receiver, and a preceding radio frequency amplifier, an automatic volume control network which comprises an electronic device connected with an element 5 providing radio frequency coupling capacity between the amplifier and rectifier, the rectifier direct current output beingutilized to regulate the conductivity of the electronic device, andthe latter functioning as Va radio frequency shunting path to ground. Still other objects of the present invention are to improve generally the simplicity and efficiency of volume control arrangements for radio receivers, and to especially provide an automatic l5 volume control circuit, of the type wherein'radio frequency coupling is regulated, which is not only reliable in operation, but economically manufactured and assembled in a radio receiver.

The novel features which I believe to be char- 20 acteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to'- both its organization and method of operation will best be understood by reference to the following description takenin connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be `carried into effect. j

In the drawing, Fig. 1 diagrammatically shows a radio receiver embodying one form of the present invention,

Fig. 2 shows a modified form of the invention. Referring now to the accompanying drawing wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a conventionalized radio receiver. The source of signals I, when the receiver is of the tuned radio frequency type, may include the usual grounded antenna circuit, and one or more tuned stages of radio frequency amplification. In the case of a superheterodyne receiver, the signal source l is to be understood asjconsist-` ing of the antenna circuit, a stage of tuned radio frequency amplification if desired, and the usual local oscillator and first detector circuit; yIn any event, whether the receiver be of the tuned radio frequency type, or superheterodyne type, there will be impressed upon the tube 2 radio frequency signal energy of a desired frequency. Y v

The tube 2 is shown as of the screen grid type, and includes an indirectly heated cathode, it being understood that the invention is not necessarily limited to this type of tube. Betweenvthe control grid electrode and the cathode is connected the 5K5 denser` C4.

circuit 3 through the desired signal frequency range, such as the broadcast-range-.andwllbe uni-controlled, as is well known to those skilled in the art, with the variable tuning condensers which may be used ahead of, or after; the tube 2.

The grid of tube 2 is maintained at a proper negative bias with respect to'thecathode by means:

of the usual biasing network 6 disposed in the grounded leg of the cathode of tube 2. The anode of tube 2 is supplied with a proper positive potential fromasource.B- (not shown), the anode be:- ing. connected` to this` source through the choke coil.L1.`

The-condenser C1. functions as the sole coupling for` radio frequency energy fromthe anode circuit of tube. 2.'.to the resonant-.input circuit 3f ofthe succeeding rectifier tube 1. Thetube I is alsoofthe-indirectly heated cathode type, and the lowradio frequencypotential side of the tuned input circuit 3 vis connected to the negative terminal. of. the source of grid biasing potential C (not shown) Thetube 1 is generally designated as rectifier, but it is tobe clearly understood that inthe case of .a tuned radio frequency receiver this tubeY functions as-the usual: detector and in the case of superheterodyne receiver the tube may functionas the -second detector. The tube 'l may, also, be. another tuned, radio 1 frequency amplifier stage.

The condenser 5. of input circuit 31', in the case of. superheterodyne reception, fixedly tunes the circuit` 3, tothe: operating intermediate frequency, andin case of tuned. radio frequency'circuits. the condenser 5 will be uni-controlledwith the condenser 5ffor tuning. the'circuit 3 through the tuningrange of the receiver.

The anode of tube. Tis-connected to Vtwoindependent paths. One of these paths includes the audio frequency coupling condenser 8 to the succeeding. stage, or.stages,,of audio frequency amplication, andthe final reproducer. They other patlrincludes in series:the coupling resistor R the. source of. anode potential B1, andthe volume controlv bias resistor R1.

The ungroundedv terminal of the biasresistor R1 is connected to the negative terminal of the source. B1, while the positive terminal ofthis potential source isconnected to one side of' the coupling resistor, R. A condenser Cz is connected in shunt with the bias resistor R1 to providethe usual time. constant: network. The condenser C3 is connected between theanode and grounded leg of: the cathode of. tube l for the customary'radio frequency by-passing' function.

Between the` two platesof condenser C1l is disposed acentralplatel?` which isgrounded. for radio frequency energy through a path which in` cludes the lead 9, the electron discharge control tube I0, and the radio frequency by-pass con- In. other. wordsfthe central plate P is grounded for radio frequency energy through the control tube I0. The control tube l0 is fedwith proper anode potential from a source B2 (not shown) through a choke L2; Thischoke should be sufficiently high in impedance so that the central plate P is not grounded through it (that is, through the choke Lz).

The control grid of tube Ill is grounded, and the cathode of the tube is connected to the ungrounded terminal of resistor R1 through a path which includes the potential source B3, and the adjustable tap Il. 'I'he positive terminal of the source B: is connected to the ungrounded side of' condenser C4, While the negative terminal of this potential source is connected to the tap II. The latter tap is adapted to be moved along the bias resistor R1 in order to predetermine the volurne levelat which the automatic volume control actionisto Ibemaintained. That is to say, the tap H can-function asa manual volume control, and at the particular setting of the tap II the signal energy' impressed upon the rectifier tube 'l will be maintained substantially constant regardless ofivariations in the signal energy collected by thereceiver.

As the. voltage of thegcathode'of' tube IU is varied-with respect to the groundedj grid, the anode to cathodeimpedance of tube I0 isr varied, and thus the resistance between the center plate P and ground is varied for radio frequency energy. Therefore, the radio frequency energy transferred acrossthe coupling condenser Ci is similarly varied, and thus thesignal energy input to the rectifier tube 'lV is regulated. The control voltage applied to the cathode of tube I0 is obtained from the anode circuit of rectifier tube l, and automatic volume control is securedsince the anode current of this rectifier tube variesas a function of the incoming signal strength.

The potential source Bz with no signalV energy impressed across the tuned circuit 3' keeps the tube l0v non-conductive, and the source B3 is chosen so as to maintain the cathode of tube I0 positively biased with respect to the grounded grid insuch a manner that the tube lll-is biased close to cut-off. However, as the signal input to the tuned circuit 3 increases', the potential drop across the bias resistor R1 increases, and the point on R1 toY which the tap I l is connected becomes morenegative. This resultsintheposi.- tive effect of the source B3 beingneutralized, with the result that the cathode of tube Ifbecomes less positive with respect to the grounded grid thereby rendering thel tube I0 more and more conductive; It will thus be seen that with increasing signal input to the tube 1, the radio frequency energy shunted to ground from the central plate P increases.

Of course, thetube 1 need not be the detector of the receiver, but may be a special volume-control tube functioning as a rectifier for the production of a direct current component which varies in accordance with signal energy fluctuations. Also, the direct current component in thevrectiiier plate circuit need not vary directly with the signal energy variations in the rectifier input circuit. For example, there may exist an inverse relationship between the two. Such a state of affairs is shown in Fig. 2.

In this figure is shown a modification of the arrangement. of Fig. 1, and only suchtubes and associated circuits are shown which are essential to an understanding of this form ofthe invention. Thus, the radio frequency amplifier 2 is shown coupled, through the condenser C to the rectifier tube '1. Inthis case, however, the usual grid leak 6' is utilized for providing rectication. In this type of rectifier, asis well known, the direct currentY component of the rectifier plate circuit varies inversely with the signal energy variations in the tuned input circuit 3'.

The audio frequency signal component is taken out of the plate circuit of the rectifier tube 1 through an audio frequency transformer M1. The control tube It, in this modification, has its cathode grounded, while its control electrode is connected to the positive terminal of source B3. The radio frequency coupling capacity which is regulated in this modification, comprises the inherent capacity existing between the screen grid of tube 2 and the anode thereof. Accordingly, the anode of tube I is connected to the screen 2 of tube 2, and the positive screen grid bias for tube 2 is obtained through the choke L2 the circuit 3 comprising the path through which the positive potential for the anode of tube IU is supplied. The choke L2 should be sufficiently high in impedance so that it does not offer any grounding effect for the screen grid 2.

It will, therefore, be seen that the modification shown in Fig. 2 comprises the grounding of the screen grid 2 of the amplifier tube 2 through a variable impedance, this impedance consisting of the control tube I0. By impressing varying direct current voltages on the grid of tube I0, the resistance of the plate circuit of this control tube is varied, and necessarily the resistance from the screen grid 2 to ground is varied. The ampliiication of tube 2 will increase due to regeneration effects within that tube circuit when a relatively high resistance is inserted between its screen grid and ground. Conversely, when a very small resistance exists between its screen grid and ground it will have a minimum of ampliiication. In all cases the voltages on control grid, screen grid, plate and cathode of the ampliiier tube 2 should be the normal values for optimum amplifier action.

It should be noted that the direct current resistance of the choke L2 in Fig. 2 should be sufliciently small to affect the voltage on the screen grid 2 to a negligible degr-ee when the tube It is `drawing its maximum plate current. In order to avoid tendency towards oscillation, the tube 2 should either be a screen grid tube with relatively low mutual conductance, or else the circuits associated with it should be designed so as not to take advantage of all of its amplifier possibility. Of course, in the arrangement shown in Fig. 2 when no signal energy is impressed on the tuned circuit 3', there will be a larg-e iiow of direct current in the plate circuit of tube l, with the result that the grid of control tube I0 will be maintained highly negative with respect to the grounded cathode. When the signal energy impressed on circuit 3 increases, the fiow of direct current will decrease, with the result that the grid of tube lll will become less negative, and the tube Ill thus becomes more conductive; that is to say, the resistance existing between the screen grid 2 and ground will then become smaller.

It is pointed out that the present invention is not merely applicable to the control of but a single amplifier stage. It is to be clearly understood that in each of Figs. l and 2 respectively the plate circuits of the control tubes I0 can be connected for controlling several stages if desired. Furthermore, while I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a radio frequency amplifier, a rectifier, said amplifier including a screen grid electrode, a control tube connecting said screen grid electrode to ground through its space current path to provide a radio frequency bypass path, and a direct current connection between the rectifier output circuit and a gain control electrode of said control tube for regulating the conductivity of said control tube sufficiently to vary the impedance of said bypass path.

2. In combination, a radio frequency amplifier, a rectifier, said amplifier including a screen grid electrode, a control tube connecting said screen grid electrode to ground, to provide a radio frequency bypass path and an adjustable direct current connection between the rectifier output circuit and a gain control electrode of said control tube for regulating the conductivity of said control tube sufliciently to Vary the impedance of said bypass path to ground.

3. In a radio receiver including at least one radio frequency amplifier tube and a following rectifier tube, said amplifier tube being of the screen grid type, a capacitative path between the screen electrode of the amplifier and the rectifier input circuit, an electron discharge tube connected between said path and ground to provide a radio frequency by-pass path to ground for radio frequencies to be transmitted from the amplifier to said rectifier, and means for varying the impedance of said by-pass tube in response to variations in value of the direct current voltage in the rectifier output circuit, said capacitative path including the inherent capacity between the screen and anode electrodes of said amplifier, and said by-pass tube having its anode connected to the amplifier screen electrode.

4. In a radio receiver including at least one radio frequency amplifier tube and a following rectifier tube, said amplifier tube being of the screen grid type, a capacitative path between the screen electrode of the amplifier and the rectifier input circuit, an electron discharge tube connected between said path and ground to provide a radio frequency by-pass path to ground for radio frequencies to be transmitted from the amplifier to said rectifier, and means for varying the impedance of said by-pass tube in reponse to variations in value of the direct current voltage in the rectifier output circuit, said by-pass tube having its anode connected to said screen electrode and its cathode grounded, the last means including a direct current connection between the control grid of the by-pass tube and the rectifier output circuit.

EDWARD T. DICKEY. 

